Automatic non-destructive case depth measuring instrument

ABSTRACT

A testing instrument for automatically non-destructively determining the hardened depth in accordance with the coercive force of a steel by periodically impressing a D.C. magnetic field externally on the steel by energizing an electromagnet from a D.C. source, measuring the magnetic flux density of said D.C. magnetic field by means of a Hall element while varying the magnetomotive force of said field, integrating the measured value of magnetic flux density, applying the integrated value to the electromagnet alternately with connection of the D.C. source to the electromagnet and reading the value of the magnetomotive force to said field at the point when the magnetic flux density has reached zero or a prescribed value.

United States Patent Kanda et a1.

[54] AUTOMATIC NON-DESTRUCTIVE CASE DEPTH MEASURING INSTRUMENT [72]Inventors: Kimio Kanda, Hitachi; Kunio Ono,

Katsuta, both of Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22]Filed: Nov. 19, 1970 [21] Appl. No.: 90,997

[30] Foreign Application Priority Data Nov. 21, 1969 Japan ..44/92954[52] US. Cl. ..324/34 R [51] Int. Cl. ..G0lr 33/12 [58] Field of Search..324/34 R, 34 H, 40

[56] References Cited UNITED STATES PATENTS 3,490,033 l/l970 Elarde..324/34 R 3,586,963 6/1971 Arrott et al. ..324/34 R FOREIGN PATENTS ORAPPLICATIONS 1,076,168 7/1967 Great Britain ..324/34 H Md/VUAL OE/MT/O/V CLEdR/NG t2 t3 t4 Aura/mm OPE/FAUO/V [451 Oct. 10, 1972 OTHERPUBLICATIONS McMaster, R., Nondestructive Test. Hands, Vol. 11, TheRonald Press; 1963; pp. 34.1, 34.2, 34.3

Primary ExaminerRobe rt J. Corcoran Attorney-Craig, Antonelli & Hill 5 7ABSTRACT A testing instrument for automatically non-destructivelydetermining the hardened depth in accordance with the coercive force ofa steel by periodically impressing a DC. magnetic field externally onthe steel by energizing an electromagnet from a DC. source, measuringthe magnetic flux density of said D.C. magnetic field by means of a Hallelement while varying the magnetomotive force of said field, integratingthe measured value of magnetic flux density, applying the integratedvalue to the electromagnet alternately with connection of the DC. sourceto the electromagnet and reading the value of the magnetomotive force tosaid field at the point when the magnetic flux density has reached zeroor a prescribed value.

10 Claims, 5 Drawing Figures 0F HARD/V555 AUTOMATIC NON-DESTRUCTIVE CASEDEPTH MEASURING INSTRUMENT BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to an automatic nondestructivecase depth measuring instrument.

2. Description of the Prior Art In measuring the hardened depth, of asteel, there has heretofore been employed a method which comprisespreparing a small test sample by breaking a piece of the steel to betested and conducting a chemical analysis on said sample. However, sucha method had the serious disadvantage that it was very inefficient andnot applicable to certain types of steel. In order to overcome suchdisadvantages, the applicant has previously proposed a case depthmeasuring instrument such as disclosed in US. Pat. Application Ser. No.20,218. The instrument disclosed in the aforesaid U.S. Pat. Applicationis designed to measure the hardened depth of a steel ky impressing a DC.magnetic field locally on the hardened case of the steel, measuring themagnetic flux density of said field by means of a magnetic field densitydetecting element while varying the magnetomotive force of said field,and reading the value of the coercive force of said field at the pointwhen the magnetic flux density has reached zero. However, the instrumenthas such practical problem that the measuring operation is cumbersomeand takes a long time, as the steps of the operation are all performedmanually.

SUMMARY OF THE INVENTION The present invention has been achieved to dealwith the above-mentioned problem. Namely, an object of the invention isto automatically and non-destructively measure the hardened depth of asteel to be tested.

Another object of the invention is to provide a steel testing instrumentwhich does not comprise mechanical elements, such as a rotary switchelement, is simple in maintenance and efficient in operation, and can beprovided in a compact form.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing thehysteresis curves of steel;

FIG. 2 is a graph showing the relationship between the hardness andcoercive force, and the depth of a quenched layer of steel; I

FIG. 3 is a diagram showing the construction of one embodiment of thepresent invention;

FIG. 4 is a diagram showing the operational characteristics of therespective switches in the instrument of FIG. 3; and

FIG. 5 is a diagram showing the waveforms at various portion of theinstrument of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 which shows themagnetic hysteresis curves of steel at a quenched portion and anunquenched portion respectively, symbol Hc, indicates the coercive forceof the unquenched portion and Hc the coercive force of the quenchedportion. In general, the coercive force is greater at the quenchedportion than at the unquenched portion.

Further, as will be seen from the experimental results of FIG. 2, thecoercive force of steel decreases with the hardness which varies as thedepth from the quenched surface increases. In FIG. 2, symbol Bmax is themaximum flux density shown in FIG. 1.

Generally speaking, a quenched layer is not distinctive in forged steelrolls, etc., as shown in FIG. 2. Namely, the steel shows a substantiallyconstant hardness to a certain depth from the surface thereof and thenthe hardness decreases gradually as the distance from the surfaceincreases.

However, since the coercive force varies with hardness, the approximatedepth of the quenched layer can be judged by penetrating a magnetic fluxdensity sufficiently deeply from the steel surface and then measuringthe mean coercive force of the flux penetrating portion.

An embodiment of the present invention, which measures hardened depthbased upon the abovedescribed principle, will be described hereunderwith reference to FIG. 3:

Referring to FIG. 3, reference numeral 1 designates a DC. exciting powersource of positive polarity, 2 a DC. exciting power source of negativepolarity, 3 a switch for the power source 1, 4 a switch for the powersource 2, 5 a coercive force measuring electromagnet having an excitingcoil 6, which is excited from said power sources 1, 2. Reference numeral7 designates a steel to be tested (hereinafter referred to simply as asample) which has a quenched layer 8. Reference numeral 9 designates amagnetic flux passing through a magnetic path-formed by the quenchedlayer 8 and a core of the electromagnet 5, 10 a magnetic flux densitydetecting Hall generator disposed in the magnetic field provided by theelectromagnet 5 for detecting the density of the magnetic flux 9, 11 apower source for supplying a control current to the Hall generator l0,12 a forward stage amplifier for amplifying an output of the Hallgenerator, and 13 an integration circuit including a resistor 14, aninput side switch 15, a main amplifier 16, a feedback capacitor 17 and adischarge switch 18 for the capacitor 17. Reference numeral 19designates a resistor for converting a current, corresponding to thecoercive force, into voltage, 20 designates a memory circuit includingamemory element 21 in the form of a capacitor and a switch 22 forclearing the value stored in the memory element 21 23 designates areverse flow preventing diode, 24 designates an indicator, 25 designatesa variable resistor for adjusting the sensitivity of the indicator 24,26 designates a resistor for setting a zero level of the indicator 24,and 27 designates a reference voltage generating circuit which applies apredetermined voltage to the zero level setting resistor 26. Referencenumeral 28 designates a switch for supplying the output of theintegration circuit 13 to the coil 6 therethrough, and 29 designates asignal generating circuit for controlling the switches 3, 4, l5, 18, 22and 28 mentioned above. The signal generating circuit 29 includes apush-button 30 and is driven as by a motor (not shown) rotatingaccording to a timed program.

The automatic case depth measuring instrument of the invention isconstructed as described above, but its essential portion is broadlycomposed of four elements consisting of the magnetizing power sources 1,2, the

magnetic field generating electromagnet 5, the integration circuit 13and the indicator 24 for indicating the hardened depth.

The operation of the case depth measuring instrument described abovewill be explained with reference to FIGS. 4 and 5. FIG. 4 shows the openand closed states of the switches 3, 4, 15, 18, 22, 28 and 30, in whichthe hatched portions indicate the period in which the pertinent switchesare held closed.

In the arrangement, when the switch 3 for the power source 1 is closed,with the switch 15 in engagement with a terminal 15b and the switch 18being held in a closed position respectively, a current I, is suppliedinstantaneously from the power source 1 to the exciting coil 6 of theelectromagnet as shown in FIG. 5, to excite said electromagnet 5. Thus,the quenched layer 8 of the sample 7 is sufficiently excited by themagnetic flux density created by said electromagnet 5, up to the point Ain FIG. 1.

Then the switch 3 is opened, whereupon a residual flux 9 remains in theelectromagnet 5, which is detected by the Hall generator 10. The Hallgenerator 10, therefore, generates an output e as shown in FIG. 5, whichis amplified by the forward stage amplifier 12 and applied to theintegration circuit 13. When the switch 15 is brought into contact witha terminal 150 and the switch 18 is opened at this time, the integrationcircuit 13 integrates the signal e and generates an output e,. Theswitch 28 is closed simultaneously with the opening of the switch 18, tosupply a current i,, of an opposite polarity to the current I, to theelectromagnet 5 by means of the output e,. The residual flux of theelectromagnet D5 is attenuated and the output e of the Hall generatordecreases. However, the integration circuit 13 continues integration aslong as the Hall generator 10 generates the output e and, therefore, thecurrent i being supplied to the exciting coil 6 of the electromagnet 5is continuously increased until the output e of the Hall generator 10becomes zero.

The magnetic flux density of the quenched layer 8 of the sample 7 movesfrom the point A towards the point B and reaches the point B in FIG. 1,whereupon the output e of the Hall generator 10 becomes zero and theintegration circuit 13 stops its integration operation.

With I, representing the current at this time and N representing thenumber of turns of the exciting coil 6, the product of I; N has a closerelation with the product of H r d wherein H represents the coerciveforce of the quenched layer 8 and d represents the depth of the same.The depth of the quenched layer of the sample 7 can be determined basedon this relation.

The current i from the integration circuit 13 at the point when theoutput e of the Hall generator 10 becomes zero, is in proportion to thecoercive force H, and the size of this current i, is obtained as aterminal voltage of the resistor 19 and stored in the memory circuit 20.

Then, the instrument is returned to the original state by opening theswitch 28, bringing the switch into contact with the terminal 15b andclosing the switch 18. Thereafter, the switch 4 is closed to supply acurrent I, of the opposite polarity to the power source 1, from thepower source 2 to the exciting coil 6 of the electromagnet 5 as shown inFIG. 5, to excite said exciting coil 6. As a result, the electromagnet 5is energized and the polarity of the residual flux in the sample 7 isreversed and further the magnetic position of the quenched layer 8 ofthe sample 7 moves from the point B towards the point C in FIG. 1. Inthis case, the output e of the Hall generator 10 is also reversed inpolarity as shown in FIG. 5 and hence the output e, of the integrationcircuit 13 is also reversed in polarity. Therefore, the current isupplied by the output e, of the integration circuit 13 to the excitingcoil 6 of the electromagnet 5 is opposite in polarity to the current I,and the residual flux 9 is attenuated by the closure of the switch 28.The output e of the Hall generator 10 drops incident to the closure ofthe switch 28, but the output of the integration circuit 13 continues toincrease until the output e becomes zero, and thus the magnetic positionof the quenched layer 8 of the sample 7 moves from the point C towardsthe point D in FIG. 1. When the magnetic position has reached the pointD, the output e of the Hall generator 10 becomes zero and theintegration operation of the integration circuit 13 is ended. The outpute of the integration circuit 13 is introduced into the memory circuit 20through the resistor l9 and thus the sum of the two measurements isstored in said memory circuit 20. Although the memory circuit 20 isshown as being composed only of a set of the memory element 21 and theswitch 22 for the sake of simplicity, it should be understood that saidmemory circuit also includes another memory element in the form of acapacitor for storing a negative output voltage of the integrationcircuit supplied thereto and a switch, in addition to the memory element21 and the switch 22. Therefore, it will be understood that the outputstored in the memory element 21 will not be ofiset. As stated above, thecoercive force H, of the quenched layer 8 of the sample 7 is measuredtwice by revering the polarity of the residual flux remaining therein.This is for the purpose of eliminating any error caused by thetemperature characteristic of the flux detecting Hall generator 10,thereby improving the accuracy of measurement.

Thus, in the memory circuit 20, the two values of voltage correspondingto the two values of coercive force measured with reverse polarities arestored in the two memory element capacitors, respectively, as describedabove, and then they are added in the same polarity so as to be fed tothe indicator 24 through the diode 23. Although the added voltage fed tothe indicator 24 corresponds to the sum of the two values of coerciveforce measured consecutively with reverse polarities, the indicator iscapable of indicating the average value of the two measurements bypre-adjusting the range of the indicator. The coercive force obtained inthe manner described above represents the coercive force of the samplesteel 7 proper when a material having a very small coercive force isused for the core of the electromagnet 5. If the electromagnet 5 isselected such that the magnetic flux density created thereby penetratessufficiently deeper than the depth of the quenched layer 8 of the samplesteel 7 from the surface thereof, the coercive force measured will bethe mean value of coercive force, and the quenched layer and a portiondeeper than that show different coercive forces respectively as shown inFIG. 2. After all, the measured coercive force wild correspond to thehardened depth.

Therefore, by previously calibrating the instrument with a test samplehaving a specific hardened depth, the hardened depth of the steel v7 tobe tested can be determined automatically and non-destructively.

Now, the timing of the opening and closure of the respective switches 3,4, 15, 18, 22 and 28, in the operation for measuring the hardened depthwith the testing instrument of the instant invention as described above,will be explained with reference to FIG. 4.

In the testing instrument of the invention, an arrangement is made suchthat when the push-button 30 is depressed at the time period t, to setthe instrument in operation for the measurement of the hardened depth ofthe sample, a control signal is generated from the switch controllingsignal generating circuit 29 at a predetermined time interval and therespective switches are automatically actuated one after another inresponse to said control signal.

Namely, the time period is the time required for erasing the memory ofthe memory circuit 20 and the switch 22 is held in a closed positionjust during this period. In the time period 1 the switch 3 is closed andagain opened, whereby the exciting current I,. is momentarily suppliedto the exciting coil 6 of the electromagnet 5. In this case, the switchis in engagement with the terminal 15b; the switch 18 is held closed andthe switch 22 is open. In the time period the switch 28 is held closedand the switch 15 is held in engagement with the terminal 15a andfurther the switch 18 is held open. Thus, the current i corresponding tothe coercive force H is stored in the memory circuit 20. Within the timeperiod the switch 4 is held closed during a certain period and thecurrent I of the opposite polarity to the current I, is momentarilysupplied to the exciting coil 6 of the electromagnet 5. In the timeperiod i the current corresponding to the coercive force is stored inthe memory circuit and in the time period I, said current or thecoercive force is indicated on the indicator 24. Further, in the timeperiod t an instruction for the next measuring operation is given to theinstrument and in the time period all the outputs stored in the memorycircuit during the preceding cycle of measuring operation are reduced tozero, providing for the next cycle of measuring operation. Thus, it willbe seen that a series of measuring operation are performed by thetesting instrument automatically, only by pushing the instruction button30 once.

As will be clearly understood from the foregoing description, accordingto the present invention the hardened depth of a given steel material,is measured by disposing a magnetic fiux density detecting element inthe magnetic field of an electromagnet which magnetizes the steel,integrating the output of the detecting element by an integrationcircuit, feeding the output of said integration circuit to the coil ofsaid electromagnet so as to offset the residual magnetic flux density ofthe steel, reducing the output of said magnetic flux density detectingelement to zero and measuring the current supplied from said integrationcircuit at the point when the output of said integration circuit and theoutput of said 'magnetic flux density detecting element are balanced,and furthermore the above-described measuring operation is effectedautomatically, according to a pre-set timed program.

Therefore, the case depth measuring instrument of the invention has suchremarkable advantages that the hardened depth of a given steel materialcan be measured automatically and non-destructively, that the operationof the instrument is highly reliable since the instrument is composed ofsolid circuits free of mechanical rotating or sliding elements, such asa servo-motor or a sliding resistor, and that, therefore, the instrumentis not only simple in construction, light in weight and small in size,but also simple in operation and maintenance. In addition, the carboncontent of steel is also measurable by measuring the coercive force ofthe steel according to the present invention.

We claim:

1. An automatic case depth measuring instrument comprising anelectromagnet for applying D.C. magnetic field to a steel to be testedlocally on a hardened case thereof from a surface of the case;

a D.C. electric power source for exciting said electromagnet;

a magnetic flux density detecting element disposed in the magnetic fieldcreated by said electromagnet for measuring the magnetic flux density ofsaid magnetic field thereby producing a first electric output signal;

an integration circuit connected to the output of said detecting elementfor integrating said first electric output signal thereby producing asecond electric output signal;

switch means for alternately supplying said electromagnet with a D.C.current from said D.C. electric power source to initially excite saidelectromagnet so as to magnetically saturate said steel andsubsequentally .with said second electric output signal to decrease saidmagnetic flux density in said steel to zero;

control means for automatically controlling the operation of said switchmeans to supply said electromagnet with the current from said D.C.electric power source and thereafter with said second electric outputsignal; and

indicator means connected to said integration circuit for indicating thevalue of said second electric output signal at the pointwhen the valueof said first electric output signal has reached zero while said secondelectric output signal is applied to said electromagnet.

2. An automatic case depth measuring instrument as defined in claim 1,said instrument further comprising a second D.C. electric power sourcefor exciting said electromagnet, the two D.C. electric power sourcesbeing selectively connected to excite said electromagnet with oppositepolarities by said switch means under control of said control means insuccessive sequences with the application of said second electric outputsignal in each sequence thereby measuring with reversed polarities thevalue of said second electric output signal at the point when the valueof said first electric output signal has reached zero while said secondelectric output signal is applied to said electromagnet, said indicatormeans being calibrated to indicate the average of said twice measuredvalue.

3. An automatic case depth measuring instrument as defined in claim 1wherein said control means includes means to control said switch meansaccording to a preset timed program.

4. An automatic case depth measuring instrument as defined in claim 1wherein said switch means includes means to disable said integrationcircuit while said electromagnet is excited by the current from saidD.C. electric power source.

5. An automatic case depth measuring instrument as defined in claim 2wherein storage means is connected between said integration circuit andsaid indicator means for storing the value representative of said secondsignal at each polarity.

6. An automatic case depth measuring instrument comprising anelectromagnet for applying a D.C. magnetic field locally on the hardenedcase surface of a steel to be tested,

a D.C. electric power source for exciting said electromagnet, magneticflux density detecting means disposed in the magnetic field created bysaid electromagnet for measuring the magnetic flux density of saidmagnetic field thereby producing a first electric 7 output signal,

an integration circuit selectively connected to the output of saiddetecting element for integrating said first electric output signalthereby producing a second electric output signal at an output connectedto said electromagnet,

indicator means for indicating the value of said second electric outputsignal,

switch means for connecting said D.C. electric power source to saidelectromagnet alternately with the connection of said integrationcircuit to said magnetic flux density detecting means and said indicatormeans to said electromagnet, and

control means for automatically controlling the operation of said switchmeans according to a preset timed program.

7. An automatic case depth measuring instrument as defined in claim 6wherein first and second D.C. electric power sources of oppositepolarity are provided for exciting said electromagnet and said switchmeans includes first and second switches controlled by said controlmeans for connecting said first and second power sources to saidelectromagnet.

8. An automatic case depth measuring instrument as defined in claim 7wherein said switch means further includes a third switch actuatable toconnect theinput of said integration circuit to the output of saiddetecting means and a fourth switch actuatable to connect said indicatormeans to said electromagnet, said third and fourth switches beingactuated simultaneously by said control means alternately with theoperation of said first and second switches.

9. An automatic case depth measuring instrument as defined in claim 8wherein storage means is connected between said integration circuit andsaid indicator means, for storing the value representative of saidsecond signal at each polarity.

10. An automatic case depth measuring instrument as defined in claim 9wherein said switch means further includes a fifth switch actuatable bysaid control means during actuation of either of said first and secondswitches to disable said integration circuit.

1. An automatic case depth measuring instrument comprising anelectromagnet for applying D.C. magnetic field to a steel to be testedlocally on a hardened case thereof from a surface of the case; a D.C.electric power source for exciting said electromagnet; a magnetic fluxdensity detecting element disposed in the magnetic field created by saidelectromagnet for measuring the magnetic flux density of said magneticfield thereby producing a first electric output signal; an integrationcircuit connected to the output of said detecting element forintegrating said first electric output signal thereby producing a secondelectric output signal; switch means for alternately supplying saidelectromagnet with a D.C. current from said D.C. electric power sourceto initially excite said electromagnet so as to magnetically saturatesaid steel and subsequentally with said second electric Output signal todecrease said magnetic flux density in said steel to zero; control meansfor automatically controlling the operation of said switch means tosupply said electromagnet with the current from said D.C. electric powersource and thereafter with said second electric output signal; andindicator means connected to said integration circuit for indicating thevalue of said second electric output signal at the point when the valueof said first electric output signal has reached zero while said secondelectric output signal is applied to said electromagnet.
 2. An automaticcase depth measuring instrument as defined in claim 1, said instrumentfurther comprising a second D.C. electric power source for exciting saidelectromagnet, the two D.C. electric power sources being selectivelyconnected to excite said electromagnet with opposite polarities by saidswitch means under control of said control means in successive sequenceswith the application of said second electric output signal in eachsequence thereby measuring with reversed polarities the value of saidsecond electric output signal at the point when the value of said firstelectric output signal has reached zero while said second electricoutput signal is applied to said electromagnet, said indicator meansbeing calibrated to indicate the average of said twice measured value.3. An automatic case depth measuring instrument as defined in claim 1wherein said control means includes means to control said switch meansaccording to a pre-set timed program.
 4. An automatic case depthmeasuring instrument as defined in claim 1 wherein said switch meansincludes means to disable said integration circuit while saidelectromagnet is excited by the current from said D.C. electric powersource.
 5. An automatic case depth measuring instrument as defined inclaim 2 wherein storage means is connected between said integrationcircuit and said indicator means for storing the value representative ofsaid second signal at each polarity.
 6. An automatic case depthmeasuring instrument comprising an electromagnet for applying a D.C.magnetic field locally on the hardened case surface of a steel to betested, a D.C. electric power source for exciting said electromagnet,magnetic flux density detecting means disposed in the magnetic fieldcreated by said electromagnet for measuring the magnetic flux density ofsaid magnetic field thereby producing a first electric output signal, anintegration circuit selectively connected to the output of saiddetecting element for integrating said first electric output signalthereby producing a second electric output signal at an output connectedto said electromagnet, indicator means for indicating the value of saidsecond electric output signal, switch means for connecting said D.C.electric power source to said electromagnet alternately with theconnection of said integration circuit to said magnetic flux densitydetecting means and said indicator means to said electromagnet, andcontrol means for automatically controlling the operation of said switchmeans according to a pre-set timed program.
 7. An automatic case depthmeasuring instrument as defined in claim 6 wherein first and second D.C.electric power sources of opposite polarity are provided for excitingsaid electromagnet and said switch means includes first and secondswitches controlled by said control means for connecting said first andsecond power sources to said electromagnet.
 8. An automatic case depthmeasuring instrument as defined in claim 7 wherein said switch meansfurther includes a third switch actuatable to connect the input of saidintegration circuit to the output of said detecting means and a fourthswitch actuatable to connect said indicator means to said electromagnet,said third and fourth switches being actuated simultaneously by saidcontrol means alternately with the operation of said first and secondswitches.
 9. An automatic cAse depth measuring instrument as defined inclaim 8 wherein storage means is connected between said integrationcircuit and said indicator means for storing the value representative ofsaid second signal at each polarity.
 10. An automatic case depthmeasuring instrument as defined in claim 9 wherein said switch meansfurther includes a fifth switch actuatable by said control means duringactuation of either of said first and second switches to disable saidintegration circuit.